red lentil soup with lemon

This soup is to die for. We first had it when Peter's sister Elsa made it for us after we had Isaac. The combination of flavors are genius and it is so satisfying. Isaac even liked it! It is from one of my favorite websites: 101 cookbooks

2 cups / 14 oz / 400 g split red lentils, picked over and rinsed well
1 tablespoon turmeric
4 tablespoons unsalted butter
fine grain sea salt
1 large onion / ~ 2 cups, diced
2 teaspoons ground cumin
1 1/2 teaspoons yellow mustard seeds
1 cup chopped cilantro
Juice of three lemons, or to taste
1 large bunch of spinach leaves, chopped
plenty of cooked (warm) brown rice, to serve
plenty of plain Greek yogurt, to serve

Put the lentils in a pot with 7 cups / 1.6 l water, the turmeric, 1 tablespoon of the butter, and 2 teaspoons salt. Bring to a boil, then lower the heat and simmer, covered, until the lentils are soft and falling apart - twenty minutes or so. Puree with a hand blender. Add more water until the soup is the consistency you like, then taste and add more salt if needed. Keep the soup warm/hot.
While the lentils are cooking, prepare the onion. In a skillet over low heat cook the onion in 2 tablespoons of the remaining butter along with the cumin and mustard seeds, stirring occasionally. When the onions have softened, roughly 10 or 15 minutes, add the cilantro and cook for a few seconds before removing from the heat. Add the onion mixture to the soup, then add the juice of the lemons, one lemon at a time - until the soup has a nice bit of tang. Also, add more salt to taste at this point if needed.
Just before serving, add the last of the butter to the skillet, when hot add the spinach and a good pinch of salt. Stir well, and cook just long enough for the spinach to collapse.
Serve by placing a scoop of rice in each bowl, then soup, spinach, and a dollop of yogurt.
Serves 6.